Electronic oscillators having their frequency determined at least in part by a capacitance have been used in many applications. Certain of these applications require highly stable operation under conditions of varying temperature. For example, certain capacitance measuring circuits incorporate such oscillators where the capacitor-to-be-measured, or sensor capacitor, controls the frequency of oscillation. Where such circuits may be used over a varied temperature, it is important that circuits provide a signal representative the capacitance of the sensor capacitor, independent of temperature. One use of such an oscillator is a pressure transducer, where the sensor capacitor has a characteristic capacitance which varies with ambient pressure.
In the prior art, temperature-stable oscillators have been developed with complex, and correspondingly expensive, compensation networks which offset nominal oscillator frequency changes with temperature. Moreover, the required degree of temperature stability for an oscillator has been an extremely important factor in determining the oscillator cost.
U.S. Pat. No. 4,603,308 describes an oscillator which is highly temperature stable and also is relatively simple, and correspondingly inexpensive, compared with most of the other prior art. However, the oscillator disclosed in that patent includes a number of amplifiers and feedback networks that require a relatively large number of circuit components. Accordingly, even the highly temperature stable, and relatively simple, oscillator of the '308 patent, still is characterized by a significant complexity and corresponding cost.
In other forms of the prior art, relatively stable oscillators include, for example, a series L-C frequency control network coupled to a cascaded capacitance, and have a common base configuration transistor amplifier coupled to the cascade of capacitance in a manner establishing an oscillator signal at the collector of the transistor.
Accordingly, it is an object of the present invention to provide an improved oscillator.
Another object is to provide an improved oscillator characterized by a relatively high degree of temperature stability.
Yet another object is to provide an improved oscillator characterized by a relatively low cost and a relatively high degree of temperature stability.